1. Field of the Invention
This invention relates to the detection of electromagnetic radiation with a multiple quantum well (MQW) superlattice structure, and more particularly to the sensing of long wavelength infrared radiation (LWIR) with an MQW superlattice in which photoexcited charge carriers are transported through the superlattice under the influence of an applied bias voltage.
2. Description of the Related Art
MQW superlattice LWIR detectors made of heterojunction materials, such as GaAs/GaxAll xAs, provide good design flexibility for spectral response. The detection of LWIR with an MQW sensor has been reported in several publications, such as Levine et al., xe2x80x9cBound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectorsxe2x80x9d, J. Applied Physics Letters, Vol. 64, No. 3, Aug. 1, 1988, pages 1591-1593; Levine et al., xe2x80x9cBroadband 8-12 xcexcm High-Sensitivity GaAs Quantum Well Infrared Photodetectorxe2x80x9d, Applied Physics Letters, Vol. 54, No. 26, Jun. 26, 1989, pages 2704-2706; Hasnain et al., xe2x80x9cGaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratingsxe2x80x9d, Applied Physics Letters, Vol. 54, No. 25, Jun. 19, 1989, pages 2515-2517; Levine et al., xe2x80x9cHigh-Detectivity D*=1.0xc3x971.010 cm {square root over (Hz)}/W GaAs/AlGaAs Multiquantum Well xcex=8.3 xcexcm Infrared Detectorxe2x80x9d, Applied Physics Letters, Vol. 53, No. 4, Jul. 25, 1988, pages 296-298.
The principal of operation for an MQW superlattice IR detector is illustrated in FIG. 1. The basic device consists of a periodic heterostructure of GaAs quantum wells 2 and AlGaAs barrier layers 4. The GaAs quantum well layers are doped with an n-type dopant, such as silicon, to provide electrons in the ground states of the wells for intersubband detection. The superlattice is sandwiched between a pair of heavily n-doped GaAs contact layers 6 and 8, with contact layer 6 functioning as an electron emitter and contact layer 8 as an electron collector during sensor operation. Ohmic contacts 10 and 12 on the opposed contact layers provide access to apply a bias voltage across the superlattice.
The thickness of each quantum well layer 2 is sufficiently small, generally about 20-60 Angstroms and most preferably about 40 Angstroms, that quantum effects are significant. The thickness of each barrier layer 4 is generally about 40-300 Angstroms, and most preferably about 140 Angstroms. The superlattice period is thus preferably about 180 Angstroms. It is generally preferred that the superlattice have about 20-30 periods. GaAs quantum well layers 2 are heavily doped n-type with a donor impurity such as Ge, S, Si, Sn, Te or Se. A particularly preferred dopant is Si at a concentration of about 1xc3x971018-5xc3x971018 cm3, and most preferably about 2xc3x971018 cmxe2x88x923. Lattice match and thermal coefficient considerations, impurity concentrations and fabrication techniques are known in the art.
Although a GaAs/AlGaAs superlattice is preferred, other materials may also be used. For example, it may be desirable to use materials such as InGaAs/InAlAs on InP, SiGe on Si, or HgCdTe. In general, superlattices fabricated from III-V, IV-IV and II-VI semiconductor materials are suitable. The MQW superlattice detectors are particularly suited for the detection of LWIR, but the sensors in general are applicable to the detection of radiation and other wavelength regimes, and no limitation to LWIR for the present invention is intended.
The potential energy barrier height of the barrier layers 4 is about 160 mev above the potential energy barrier height of the quantum wells 2 for GaAs/AlGaAs. For LWIR with peak detection of about 12 microns, the energy gap between the bound state and the excited state for electrons in the quantum wells is about 100 mev, with the first electron excited state in the quantum wells lying above the conduction band edge of the barrier layers.
Incident infrared photons excite electrons from the quantized baseband of the wells to extended excited states in a continuous conduction subband, which has an energy level greater than the conduction band floor for the barrier layers. The excited electrons are then accelerated towards the collector contact 8 by an electric field created by an externally applied bias voltage source Vb. Under normal sensor operating conditions, the bias voltage causes the mean-free path of electrons in the subband 8 to be sufficiently large for the electrons to travel under the applied field through the superlattice, producing a photocurrent that is measured as an indication of the magnitude of incident radiation. An ammeter 14 can be inserted in the circuit between contact layers 10 and 12 for this purpose.
The sensitivity of an MQW superlattice infrared detector can be severely limited by high levels of dark current. This current consists primarily of electrons which tunnel through the intervening barrier layers 4 between the ground states of adjacent quantum wells 2. The tunneling current can be reduced by increasing the widths of the barrier layers 4. However, any such increase in the barrier layer width reduces the device""s radiation hardness, which is inversely related to its thickness.
An improvement upon the detector as described thus far is disclosed in pending U.S. patent application Ser. No. 07/457,613, filed Dec. 27, 1989 by Sato et al., xe2x80x9cDark Current-Free Multiquantum Well Superlattice Infrared Detectorxe2x80x9d, and assigned to Hughes Aircraft Company, the assignee of the present invention. Under this approach the barrier layers 4 are kept thin, but a thicker (generally about 800-3,000 Angstroms) tunneling current blocking layer is provided at the end of the superlattice in the path of the tunneling electrons. The blocking layer 16, which is preferably formed from the same material as the barrier layers 4, eliminates most of the tunneling current component of the photodetector""s dark current. This in turn allows the individual barrier layers 4 to be made thinner, thus enhancing the detector""s quantum efficiency and increasing its radiation hardness.
The prior MQW superlattice IR detector illustrated in FIG. 1 requires both a relatively large number of quantum well/barrier layer periods, generally 20-30, and a relatively high bias voltage of about 3 volts to obtain sufficiently high quantum efficiency. The large number of superlattice periods can cause detector failure if the detector is operated in a harsh radiation environment. Furthermore, since the number of electrons that are absorbed in the superlattice before reaching the collector contact increases with the initial distance of the electrons from the collector contact, the portions of the superlattice that are distant from the collector contact reduce the overall quantum efficiency. The relatively high bias voltage requirement can lead to a high noise-to-signal ratio and large power consumption.
The present invention seeks to provide an MQW superlattice radiation detector that is operable with a substantially reduced bias voltage compared to prior detectors, and has a higher quantum efficiency, lower noise level and lower power consumption.
These goals are achieved with a radiation detector in which a plurality of MQW superlattice detectors are structurally formed in a unitary stack, but are electrically connected in parallel. Electrical contact layers are provided between each adjacent pair of superlattices and at opposite ends of the stack, with the adjacent superlattices sharing common contact layers between them. The number of quantum well/barrier layer periods per superlattice can be reduced to about 20-30 divided by the number of superlattices in the stack. This reduces the required bias voltage across the parallel-connected superlattices by a corresponding amount, with an accompanying increase in quantum efficiency and a reduction in noise and power consumption.
The different superlattices can either be similar in structure and composition, or the thickness and composition of their barrier layers can be varied so that the superlattices are sensitive to different radiation wavebands; a multi-color detection system is possible with the latter approach.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: